1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the zoom lens, and more particularly, to a photographing system for a digital camera, a video camera, a TV camera, a monitoring camera, a silver-halide film camera, and the like.
2. Description of the Related Art
In recent years, an image pickup apparatus (camera) using a solid-state image pickup element, for example, a video camera or a digital still camera has been reduced in size and improved in function. An image pickup optical system used for the image pickup apparatus is required to be a zoom lens which is wide in field angle, high in magnification (high in zoom ratio), large in aperture ratio, and small in size. Such a kind of camera includes various optical members such as a low-pass filter and a color correction filter, which are provided between a rearmost lens and the image pickup element, and hence the image pickup optical system used for the camera is required to have a relatively long back focal length. Further, an image pickup optical system used for a color camera including an image pickup element for color image is expected to have an excellent image-side telecentric characteristic in order to prevent color shading.
There is known a negative lead type zoom lens in which lens units having negative refractive powers are provided on the object side to attain a wide-field angle zoom lens with a relatively long back focal length. A known example of the negative lead type zoom lens is a four-unit zoom lens which includes four lens units with negative, positive, negative, and positive refractive powers in order from the object side to the image side and has a zoom magnification of approximately 5 (Japanese Patent Application Laid-Open No. 2006-208889). There is known another negative lead four-unit zoom lens of the same zoom type, in which respective lens units are moved during zooming (U.S. Pat. No. 5,710,669).
In general, the negative lead type zoom lens is advantageous in achieving a wide field angle but has a significantly asymmetrical lens configuration. Therefore, for example, in the case of the four-unit zoom lens in which the respective lens units move during zooming, aberrations significantly vary because of change in asymmetrical lens configuration, and hence it is very difficult to obtain high optical performance over the entire zoom range. In order to downsize the entire system of the four-unit zoom lens described above and obtain high optical performance over the entire zoom range at the wide field angle, it is important to suitably set the refractive powers of the respective lens units, the lens configuration, and a movement condition of the lens units during zooming. For example, it is important to suitably set a refractive power of a third lens unit which affects a size of a first lens unit and set a movement condition of a second lens unit (serving as main zooming lens unit) during zooming. It is more important to set a lens configuration of the second lens unit and the refractive powers of the respective lens units in a balanced manner.